Observable cosmological vector mode in the dark ages

Abstract

The second-order vector mode is inevitably induced from the coupling of first-order scalar modes in cosmological perturbation theory and might hinder a possible detection of primordial gravitational waves from inflation through 21cm lensing observations. Here, we investigate the weak lensing signal in 21cm photons emitted by neutral hydrogen atoms in the dark ages induced by the second-order vector mode by decomposing the deflection angle of the 21cm lensing signal into the gradient and curl modes. The curl mode is a good tracer of the cosmological vector and tensor modes since the scalar mode does not induce the curl one. By comparing angular power spectra of the 21cm lensing curl mode induced by the second-order vector mode and primordial gravitational waves whose amplitude is parametrized by the tensor-to-scalar ratio r, we find that the 21cm curl mode from the second-order vector mode dominates over that from primordial gravitational waves on almost all scales if r 10 −5 . If we use the multipoles of the power spectrum up to ℓmax = 10 5 and 10 6 in reconstructing the curl mode from 21cm temperature maps, the signal-to-noise ratios of the 21cm curl mode from the second-order vector mode achieve S/N ≈ 0.46 and 73, respectively. Observation of 21cm radiation is, in principle, a powerful tool to explore not only the tensor mode but also the cosmological vector mode.

Inflation and Early Dark Energy with a Stage II Hydrogen Intensity Mapping Experiment

“…We have also indicated the levels at which other effects begin to dominate the primordial signal. For curl lensing, vector perturbations generated at second order by primordial scalar perturbations produce the dominant signal if r 10 −5 [339,340]. Contaminants in the tidal fossil estimator have not been extensively investigated, but tensor 31.…”

Numerical solutions to Einstein’s equations in a shearing-dust universe: a code comparison

“…However, vector modes may also be sourced through nonlinear processes (e.g. [77][78][79][80][81][82][83]), potentially interfering with our ability to measure phenomena such as primordial gravitational waves of sufficiently small amplitude [84]. They can also produce a gradient in the matter density field [85].…”

Numerical solutions to Einstein's equations in a shearing-dust Universe: a code comparison

“…However, vector modes may also be sourced through nonlinear processes [e.g. [74][75][76][77][78][79][80], potentially interfering with our ability to measure phenomena such as primordial gravitational waves of sufficiently small amplitude [81]. They can also produce a gradient in the matter density field [82].…”